Patch clamping method are used to characterize ionic conductances of phototactic signal processing in Chlamydomonas, a unicellular ciliated eukaryote. The response to light in this organism is a hyperpolarizing receptor potential exhibiting homologous form to that found in the vertebrate and other "modified cilium" photoreceptors. The input to sensory processing operations is a signal (?cGMP) from a bovine-like rhodopsin coded by a gene with conserved sequence homology to bovine rhodopsin cDNA. Functional comparison of light-coupled ionic conductances in Chlamydomonas to conductances already studied in rods or cones would determine whether the same homology that exists at the receptor level also extends to the proteins which rhodopsin couples to in signal processing, and whether these signal processing channel elements are functionally homologous to those in the rod inner segment (RIS). Proof of homology of signal processing environments in Chlamydomonas to vertebrate photoreceptors would interest a wide audience of visual physiologists and geneticists because of physiologic, biochemical, and genetic accessibility. A critical advantage of the Chlamydomonas system over vertebrate photoreceptors is that its genetics is well established with mutants available (or readily generated) along the entire input- output pathway. Chlamydomonas may then permit a genetic dissection of sensory processing similar to neurogenetic studies in paramecium and Drosophila. In a homologous sensory processing environment cDNA technology could be exploited to clone genes for the channels or enzymes governing unicellular vision and these probes could be used to map sites for similar functions in the human genome. Conversely, cDNA probes believed to be responsible for human photoreceptor processing defects (e.g. in retinitis pigmentosa (RP) or congenital nyctalopia (CN)) could be screened for physiological or structural effects in an accessible homologous system. Another advantage of Chlamydomonas over rods or cones is that it is freely- behaving organism without the connections to other cells that complicate studies of input-output relationships in rods or cones in the intact retina.